Stability and traction are important to the operation of motorized land vehicles. Without either, a vehicle may spinout or tip over, causing damage and injury. Stability allows the vehicle to resist inertia during a change in the magnitude or direction of motion. While traction allows the vehicle to grip the surface upon which it moves.
Stability is commonly controlled by the driving skill of the vehicle driver and by proper vehicle weight distribution, or more specifically, by the position of the vehicle's center of gravity (CG) relative to the ground. CG is a virtual point through which inertial forces appear to act on the vehicle. Adjusting the CG closer to the ground tends to improve the vehicle's traction, and helps reduce the required driver skill. Many modern vehicles are designed with a differential in order to maintain stability while maneuvering. However, other vehicles may not use a differential for reasons of cost and complexity.
A differential is a structure usually coupled to an axle and allows the wheels to rotate at different rates. Without a differential, wheels attached to a single axle must rotate together at the same speed. This presents a problem in a turn, since the inside wheel (traveling on a smaller arc) tends to turn a lot slower than the outside wheel (traveling on a larger arc). In order to effectively complete the turn, the inside wheel would either need to skid over the road surface, or must lift while turning.
Go-carts, for example, typically lack a differential. The typical racing go-cart is a relatively simple motorized vehicle large enough to accommodate only one individual, consisting of a tubular frame with a generally single-cylinder engine. A go-cart frame is usually formed from segments of steel tubing rigidly welded together, and the rear axle is connected to this frame by means of rigid bearing hangers. Each of the front wheels are coupled to individual stub axles that independently rotate around some type of pin, which is further coupled to the steering mechanism.
While turning, centripetal force causes a weight shift away from the center of the turn. It is this weight shift that helps cause the inside rear wheel to lift. If the inside wheel does not sufficiently lift of the road surface, it causes the rear axle to lock and make the vehicle sluggish. If the inside wheel lifts too much, the go-cart may change direction too rapidly, subsequently causing the vehicle to spin or slide. The amount of traction, and hence overall performance of the vehicle can be modified by altering the stiffness of the axle, and hence its tendency to lift while maneuvering. Stiffness refers to the rigidity of the axle. That is, the amount of deflection for the axle produced by a given force. In general, deflection depends on the length of the axle, its cross-sectional shape, the material, where the deflecting force is applied, and how the axle is supported. In a turn, the degree of axle deflection for any point along the longitudinal axis tends to increase toward the inside wheel. However, structures coupled along the axle, such as the wheel hubs, tend to decrease the deflection, since they act to support the axle.
A common way to modify axle stiffness is to replace the entire axle. In general, the stiffer the axle, the greater the wall thickness. Go-cart manufacturers typically make axles of a soft, medium and hard stiffness. The use of medium stiffness is most common. The hard axle is used when the weather is cold, in slippery track conditions, or when rules mandate the use of harder compound tires. The soft axle is used if conditions are extremely “grippy”, or where there is excess rubber build up.
Replacing axles can be time consuming and problematic. Originally a recreational pass time, go-cart racing has become an almost semi-professional sport with organized teams and expensive stylized go-carts. A competitive race normally comprises a set of laps, wherein wining is often determined by a fraction of a second. Removing an axle often requires the detachment of the rear wheel hubs, a brake disk, and a drive sprocket. These components that are often difficult to quickly remove in a safe manner, without the use of multiple tools. In practice, since axle replacement often takes over 10 minutes, it is usually done only between races. However, racing conditions can often change during the race. For instance, a decrease in temperature or a rain shower can reduce the tackiness of the tires, causing a reduction in tire traction.
FIG. 1 illustrates a simplified diagram of a go-cart chassis 100. Front axle 110, is normally comprised of a fixed width wheelbase, and provides a steering mechanism for the vehicle. That is, wheels 112a-b rotate left or right in response to corresponding movements in steering column 114. Rear axle 104 is often connected to an engine, through a drive chain or belt through drive sprocket 105. Rear wheels 106a-b are commonly attached to rear axle 104 through some type of clamp assembly 102a-b. In a common method, clamp assembly 102 securely clamps the wheel hub of wheel 106 to a position along axle 104. Rear axle 104 further comprises a brake disk 103, coupled to the braking system, and a drive sprocket 105. Replacing rear axle 104 first requires the driver to stop the engine and exit the go-cart. Removing clamp assembly 102a-b, brake disk 103, and drive sprocket 105. Sliding out the old axle. Replacing the new axle. And finally, reattaching clamp assembly 102a-b, brake disk 103, and drive sprocket 105. The driver can then re-enter the go-cart and re-start the engine.
Referring to FIG. 2, a simplified diagram of the go-cart chassis 100 of FIG. 1 during a turn is shown. In this example, the go-cart is turning clockwise 204 (or to the right from the driver's perspective). As the go-cart turns, the vehicles inertia 206 imposes a centripetal force. This causes a weight shift away from the center of the turn, lifting the inside rear wheel 106a of rear axle 104 by a clearance 208. It is clearance 208 that helps prevent the rear axle from locking, reducing the vehicles performance. Reducing the stiffness of rear axle 104 can increase clearance 208 in a turn, whereas increasing the stiffness can reduce clearance 208.
Referring to FIGS. 3A-C, simplified diagrams of rear axle 100 of FIG. 1 during a turn are shown with varying degrees of rear axle stiffness. In these examples, the go-cart turns clockwise (or to the right from the vehicle driver's perspective), the vehicles inertia 206 imposes a centripetal force. This causes a weight shift away from the center of the turn. In FIG. 3A, a soft rear axle 104a is shown lifting the inside rear wheel 106a of by a clearance 208a. In FIG. 3B, a medium rear axle 104b is shown with the same inertial force 206, lifting the inside rear wheel 106a by a smaller clearance 208b. In FIG. 3C, a hard rear axle 104c is shown with the same inertial force 206, lifting the inside rear wheel 106a by a still smaller clearance 208c. 
An alternative solution to replacing the rear axle is increasing the effective wall thickness by the use of an inner stiffening cylinder. This structure can be inserted into the open longitudinal cavity of the rear axle. In a common method, the inner stiffening cylinder can be inserted into the rear axle during a race while the engine is running and the driver is still on the vehicle. The inner stiffening cylinder, is usually about the same length as the rear axle, and is first inserted than then expanded to firmly contact the rear axle's inner surface, creating a friction lock. However, this method can also be problematic. Since the rear axle does not normally have end caps, the inner cylinder can vibrate lose during a race, and slide (or in some cases fly) out of the rear axle, potentially causing injury and damage.
Referring to FIGS. 4A-C, simplified diagrams of rear axle 100 of FIG. 1 during a turn are shown with an inner stiffening cylinder. As in FIGS. 3A-C, the go-cart turns clockwise (or to the right from the vehicle driver's perspective), the vehicles inertia 206 imposes a centripetal force. This causes a weight shift away from the center of the turn. In FIG. 4A, soft rear axle 104a of FIG. 3A is shown lifting the inside rear wheel 106a of by a clearance 208a. In FIG. 4B, soft rear axle 104a comprises an inner stiffening cylinder 406 further creating an effective stiffness substantially equivalent to medium rear axle 104b of FIG. 3B. In this configuration, the same inertial force 206 lifts the inside rear wheel 106a by a smaller clearance 208b. In FIG. 4C, soft rear axle 104a comprises an inner stiffening cylinder 408, with a wall thickness greater than that in FIG. 4B, and further creating an effective stiffness substantially equivalent to hard rear axle 104c of FIG. 3C. In this configuration, the same inertial force 206 lifts the inside rear wheel 106a by a still smaller clearance 208c. 
In the discussions that follow, the term “tighten” is employed herein to discuss moving a fastener into a securing structure. Likewise, the term “loosen” is employed herein to discuss moving a fastener out of a securing structure. The term screw should be understood to apply to other types of fasteners, such as bolts, clips, and pins. Furthermore, the terms “soft,” “medium,” and “hard” refer to relative and not absolute degrees of stiffness. That is, “medium” is stiffer than “soft,” and “hard” is stiffer than both “soft” and “medium.” A structure may be presumed to be “hard” in comparison to other structures in one context, yet may be determined as a “soft” structure in another context.
In view of the foregoing, there is desired an apparatus for optimizing the stiffness of an axle.